The present invention relates to modulator systems, and more particularly to a self-adjusting I-Q modulator system that operates over a wide range of carrier frequencies.
I-Q modulators are used to simultaneously amplitude and phase modulate a carrier to produce QAM or other forms of modulation. The basic block diagram of an I-Q modulator is shown in FIG. 1. Phase shifters are used to modify the carrier phase applied to respective multiplier blocks so that the I channel phase is in quadrature with the Q channel phase. The modulators used may be any type of four-quadrant voltage controlled multipliers, but it is typical to use double balanced hybrid ring mixers or double balanced mixer ASICs for the function.
It is relatively simple to construct this subsystem if the carrier frequency is fixed or operates only over a limited frequency range. When operated over a wide range of frequencies the major problem is maintaining the quadrature relationship of the carrier at the input to the two modulators. Typical phase shift networks have either transmission line or reactive elements. In either approach phase is a strong function of frequency and circuit elements must be readjusted to maintain quadrature operation at each frequency if even modest results are to be obtained. Other, more broadband, phase shift networks, such as semi-infinite RC networks or 3 dB hybrid couplers, are sometimes used to increase the frequency range. While their outputs are relatively close to quadrature over one or more octaves, they too require circuit adjustment if excellent results are to be obtained.
Three of the best examples of prior art are U.S. Pat. Nos. 4,717,894, 5,119,399 and 5,694,093. The '894 patent combines the use of reference offset compensation, variable gain in the I and Q channels, calibrated I and Q sources and an RF detector to measure and adjust the I-Q modulator's operating parameters. This concept is extended slightly by the '399 patent. Both patents use a block diagram similar to FIG. 2. The calibration procedure is roughly as follows:                I and Q inputs are set to the zero position. The I and Q offsets are iteratively adjusted to null the Modulated Carrier Output signal as measured by the output detector.        The I input (only) is then connected to the voltage reference signal. The resulting signal is measured and compared with the result obtained when the voltage reference is connected to only the Q signal.        The variable gain is adjusted until the same signal amplitude is measured for each channel, alone, at the detector's terminal.        Phase is adjusted by applying the standard I and the standard Q signal simultaneously. The output signal amplitude is compared between when the polarity of the I signal is reversed.        The phase to the modulators is adjusted until the output signal amplitude is the same with either polarity of the I channel signal.The '399 patent is differentiated from the '894 patent by using a more complicated system of reference voltages.        
The '093 patent describes an I-Q GaAs modulator ASIC that adjusts itself for orthogonality over a 16:1 frequency range. The basic scheme is shown in FIG. 3. By using integrated circuit technology the values of the two capacitors C are created essentially equal. Likewise the two resistors R are essentially equal in value by using identical FET transistor circuits in each. The amplifier senses the detector outputs and adjusts the values of the resistors R until the output voltage from each detector is equal. This happens only when the value of the resistor R is equal to the reactance of the capacitor C at the operating frequency. At this value of resistance one network advances the carrier phase by 45° while the other retards it by 45°, thus bringing the two carrier signals into a quadrature relationship. The offset and gain parameters of this I-Q modulator are adjusted by similar schemes as shown in the previous patents with components external to the ASIC.
All three of these patents adjust the operation of an I-Q modulator by making only magnitude measurements of the resulting signal. What is desired is the ability to improve the adjustment of the operating parameters of the I-Q modulator by using the measurement of both amplitude and phase for a full waveform of the modulated carrier output.